Slow decreases of ionic conductance across neuronal cell membranes, which generate slow postsynaptic potentials, can increase the effectiveness of synaptic transmission. Slow conductance decreases of sufficient magnitude increase the amplitude of monosynaptic of fast EPSP's in B cells of the bullfrog sympathetic ganglia. By this postsynaptic mechanism, activation of one synaptic pathway can cause a long-lasting increase in transmission efficacy at other synapses. Such postsynaptic mechanisms may be important for synaptic integration and control of neuronal interaction.